1. Field of the Invention
The present invention relates to a manufacturing method for a semiconductor device provided with a diffraction grating, and particularly to a manufacturing method for a semiconductor device capable of maintaining a desired shape of the diffraction grating.
2. Background Art
A resonator structure of semiconductor laser may have a diffraction grating near an active layer to unify the emission wavelength. This diffraction grating has a periodic structure for selectively oscillating a specific wavelength emitted from the active layer. The resonator structure having the diffraction grating is called distributed feedback.
In general, the diffraction grating has a rectangular periodic structure formed by forming a layer called the guide layer and etching part of the guide layer. The shape of the above-mentioned diffraction grating is one of parameters for deciding an optical coupling coefficient indicating light wave-guided through the active layer and the degree of coupling of the diffraction grating. Here, the optical coupling coefficient is an important parameter for semiconductor laser because it contributes to the oscillation wavelength of the semiconductor laser and transmission characteristics. It is desirable that the diffraction grating be formed in a desired shape (a rectangle in the above-mentioned example) stably.
A forming method of a typical diffraction grating is as follows. First, a layer called an optical guide layer to be formed as a diffraction grating is formed. Next, a layer called a cap layer is formed on the optical guide layer. After that, an opening portion is formed in part of the optical guide layer and the cap layer using photo engraving and etching techniques. The formation of the opening portion is performed in such a manner that the optical guide layer forms the diffraction grating. Next, burying growth is performed to bury the above-mentioned opening portion, completing the diffraction grating.
In general, the substrate temperature during the above-mentioned burying growth is equivalent to the growth temperature of the cap layer and the guide layer. In other words, since the substrate temperature during the burying growth is higher enough than a temperature at which the cap layer and the optical guide layer cause mass transport, the mass transport is active. The shape of the diffraction grating of the optical guide layer can deform during the burying growth. Therefore, for example, there is a problem that the shape of the diffraction grating to be formed in a rectangular shape can vary during the burying growth to affect the optical coupling coefficient so that a desired laser oscillation wavelength and transmission characteristics cannot be obtained.